1. Field of the Invention
This invention relates to the treatment of a reaction mixture of oxygen-containing gas and unsaturated normal C.sub.4 hydrocarbons to form maleic anhydride over a catalyst having a novel composition and to the enhancement of the selectivity and stability of the catalyst by the control of the composition of a carrier gas component of said oxygen-containing gas.
2. Description of the Prior Art
The production of maleic anhydride by the oxidation of a hydrocarbon is complicated by the side reactins leading to the formation of carbon monoxide, carbon dioxide, and miscellaneous organic compounds. Benzene continues to be the preferred hydrocarbon feed material under appropriate feedstock price conditions. At times when the price of benzene is relatively high, compared to the price for reasonably pure normal unsaturated C.sub.4 hydrocarbons, there can be economic advantages in the production of maleic anhydride from unsaturated normal C.sub.4 hydrocarbons even when the yields are less than sometimes attained with benzene. Mixtures containing butadiene, butene -1, cisbutene -2, trans-butene -2, as well as mixtures containing unsaturated C.sub.5 hydrocarbons comprising cyclopentadiene, pentenes, and pentadienes can be used instead of unsaturated C.sub.4 hydrocarbons. MA (i.e. maleic anhydride) has been catalytically prepared by oxidation of normal butane at temperatures higher than those employed for conversion of unsaturates and at generally lower selectivities. The production of MA from butane can be deemed to be as different from making MA from unsaturated normal C.sub.4 hydrocarbons as is the making of MA from benzene.
A considerable variety of catalysts have been found to have some activity for the oxidation of normal butenes to MA. However, the relative proportions of carbon dioxide, carbon monoxide, and undesired organic impurities resulting from the use of any of the many previously proposed catalysts has been high enough to jeopardize the profitability of MA production. A high proportion of the literature descriptions of catalysts for production of MA concern catalysts which were sufficiently unsatisfactory that they are conveniently designated as abandoned scholarly investigations.
In the production of MA from n-butenes, there has been a long term preference for catalysts featuring vanadium oxide, as exemplified by Kerr in U.S. Pat. No. 3,156,705. Multiple metal catalysts have been proposed by Milberger et al in U.S. Pat. No. 3,907,834, and Otaki et al in U.S. Pat. No. 3,992,419. However, these catalysts have not provided yields which could be satisfactory commercially under current engineering standards. In a series of Ueeda patents including U.S. Pat. Nos. 3,906,008, 3,975,407, and 4,003,920, all the disclosures of which are deemed here reiterated, 1-butene is oxidized to MA over a catalyst featuring the oxides of tungsten and phosphorus and modified by an appropriately selected metal oxide promoter. Some persistent, proficient attempts to reproduce the reported selectivities, yields and conversions of selected examples of said Ueeda patents have been unsuccessful. Furthermore, relatively poor yields for preparing MA from n-butenes have been reported by M. Ai in two articles entitled "The Activity of WO.sub.3 Based Mixed Oxide Catalysts", J. of Catalysis 49, 305-312 and 49, 313-319 (1977), concerning modified phosphotungstic catalysts of the Ueeda type. As explained by Ueeda in U.S. Pat. No. 3,906,008, and further illustrated in comparative example 2 of U.S. Pat. No. 4,003,920, the presence of vanadium in their phosphotungstic type of catalyst led to unsatisfactory yields of M.A.
Notwithstanding the persistent effort at improving catalyst, the chemical industry has continued to seek a catalyst having acceptable stability, conversion and selectivity for producing maleic anhydride from unsaturated aliphatic hydrocarbons.